The present invention relates to barge-mounted cranes utilized for lifting extremely heavy loads, such as in bridge building. More generally, it relates to increasing the stability of vessels, such as barges, upon which large external moments are applied, such as lifting heavy weight over the side of the vessel by a crane mounted thereon.
Cranes, such as of the boom type, have long been mounted on barges, floating platforms, or other boats and utilized for various purposes, such as loading and unloading of ships and barges, ship repair, reclaiming sunken ships, and bridge building. The size of crane barges must be limited, especially the width, to permit passage through locks, canals and other narrow inland waterways. Similarly, other economic or design constraints may limit the length, breadth or draft of any type of floating vessel.
In use of crane barges, extremely heavy loads, several times the unloaded displacement of the barge, sometimes need to be so lifted to considerable horizontal distances from the barge, particularly for bridge building. When utilizing a barge-mounted crane for lifting these heavy loads, such as a large section of a bridge weighing hundreds of tons, the barge may list or trim severely, and the draft will increase substantially. If the inclination, especially listing, should become too great, the crane barge may become unstable, and could capsize rather than right itself. At lesser angles the deck of the barge may be an unsafe working surface. Such tilting of the barge may make control of the crane uncertain. Likewise, any type of boat or vessel which normally has positive stability, conventionally defined by having its center of gravity beneath its metacentre, may become unstable when a large external moment is applied, such as caused by improper loading or wave action.
The truss-like boom of a barge-mounted crane, for example, which may be several times longer than the length of the barge, has little strength except in its plane. Since the crane is normally positioned aft of the center of flotation of the barge to facilitate lifting over the end of the barge, lifting over the side of the barge may cause the crane boom to tilt aft from its normal substantially vertical plane due to aft sinkage of the barge. Similarly, lifting over a corner of the crane barge, rather than directly over an end or side, may cause a similar condition due to simultaneous listing and trimming of the barge. Also, the crane boom is subject to high wind loads normal to the vertical plane of the boom, which effects sideward shear on the boom and a moment at the connection of the boom to the barge, causing the barge to list or become out of trim and the plane of the boom to tilt from vertical. In these cases when the boom tilts from vertical, any suspended load presents a sideward component of force which is likely to collapse the boom. Tilting out of the vertical by one degree or even less may endanger the crane. Other types of vessels may be endangered by unsymmetrical loads, whether or not accompanied by wind forces.